Cancer cells divide and multiply much faster than most normal cells — and that's typically how they do their harm. Uncontrolled cell growth can lead to masses of cancer cells called tumors, or to a situation where healthy cells are crowded out and can no longer do their jobs efficiently.

Chemotherapy (or "chemo") and radiation, the two most common types of cancer treatment, work by destroying these fast-growing cells. But other types of fast-growing healthy cells (such as blood and hair cells) also can be damaged along with cancer cells, causing adverse reactions, or side effects.

Side effects can range from fatigue and flu-like symptoms to hair loss and blood clotting problems. Because it's hard for doctors to predict how a child's body will react, all kids who undergo treatment for cancer are very closely monitored. Doctors weigh the amount and severity of side effects against the benefits of treatments.

Fortunately, most side effects are temporary — as the body's normal cells recover, these problems gradually go away.

What to Expect

Side effects vary from child to child: Some can be merely unpleasant, while others can be much more serious; some show up right away, while others develop over time; and some kids have just a few, while others have many over the course of treatment.

Chemo and radiation produce similar side effects. Chemo's side effects depend on the type of drug used, the dosage, and a child's overall health. These effects are more likely to affect the whole body.

Radiation's side effects, on the other hand, tend to be more limited to the area that is being treated. However, they do still depend on the dose of radiation given, the location on the body, and whether the radiation was internal or external.

Here are some of the side effects associated with these cancer treatments, and how to manage them:

Fatigue

This is the most common side effect of both chemotherapy and radiation. Even the most active kids are likely to find themselves exhausted and perhaps even a little "foggy-headed" during treatment — and possibly for a while afterward. This is normal. Encourage your child to scale back on activities and to rest as much as possible. Once treatment is over, your child's energy should return.

Flu-like Symptoms

Some cancer medications appear to trigger the body's normal inflammatory response, producing flu- or cold-like symptoms, such as a runny nose, chills, and cough. Drinking plenty of fluids can help clear excess mucus. Also ask your doctor which, if any, over-the-counter medicines might be helpful.

Pain

Some chemo drugs are known to cause headaches, muscle pains, stomach pains, or even temporary nerve damage, which can result in burning, numbness, or tingling in the hands and feet. If this happens, your doctor can prescribe medications that can help. Never use over-the-counter or herbal medicines without your doctor's consent, though, since these can interact with the chemo drugs.

Mouth, Gum, and Throat Sores

Both chemo and radiation (specifically to the head and neck) can lead to mouth sores, sensitive gums, an irritated throat, and an increased risk of tooth decay. The doctor may prescribe a mouth rinse to reduce irritation. Soft, cool foods might be easier to eat, and high-acid foods and juices (like oranges or tomatoes) should be avoided. Regular dental checkups are important too.

Gastrointestinal Problems

Many types of chemo drugs are known to cause nausea, vomiting, loss of appetite, constipation, or diarrhea. Medications are available to prevent or alleviate a lot of these symptoms. It's also common for kids to find that their taste preferences change while on chemo (they can't tolerate certain smells or textures, for example).

If your child's appetite wanes, try offering several small servings of something rather than three large meals. Also concentrate on keeping your child hydrated with water, juices, and broths.

Gastrointestinal symptoms related to radiation tend not to be as severe as those brought on by chemo, except in children who receive radiation to the pelvis or abdomen.

Skin Changes

Chemo drugs commonly cause rashes, redness, and other types of skin irritation — especially if your child has had radiation prior to the chemo (this is called "radiation recall"). Radiation alone can cause similar symptoms, along with blisters, peeling, and swelling, in the area of treatment.

Wearing loose, soft cotton clothing may help with the discomfort. Your doctor might also recommend or prescribe creams or ointments. Because the affected area can be more sensitive to the sun for a while after treatment, your child should always wear sunscreen with an SPF of at least 30 whenever going outdoors.

Weight Changes

Some kids may experience weight loss or weight gain. It's common for those taking steroids to have an increased appetite and gain weight in unusual places, like the cheeks or back of the neck. Other kids might have decreased appetites or trouble keeping food down (especially if they're feeling nauseous after chemo).

If you're concerned about your child's weight, talk to your doctor about how to help your child maintain a healthy weight based on his or her medical needs.

Hair Loss

During chemo, hair thinning and hair loss may occur all over the body. Radiation therapy to the head and neck may cause hair loss in that area; however, radiation anywhere else will not cause the hair on the head to fall out.

Though some kids take hair loss in stride, others find it very traumatic. Assure your child that the hair will grow back — though it might be a slightly different color or texture. In the meantime, many kids choose to wear baseball hats, bandanas, scarves, or wigs.

Prior to treatment, some kids get shorter haircuts, as it can be less traumatic to see shorter strands of hair fall rather than long ones.

Kidney and Bladder Problems

Some chemo drugs are known to be hard on the kidneys, making them less able to function well. Your child will have frequent blood tests to monitor kidney function. Staying well hydrated can help. Tell the doctor if your child has bloody urine or any problems urinating.

Anemia

Both chemotherapy drugs and radiation can destroy all types of healthy blood cells and adversely affect the body's production of new ones. Low levels of red blood cells (RBCs, the cells that carry oxygen) can lead to anemia, which causes fatigue, paleness, shortness of breath, and a fast heartbeat.

Your child's blood will be drawn frequently throughout treatment to monitor the levels of these cells. If RBCs are too low, donor cells may be given through a blood transfusion.

Blood Clotting Problems

Cells that help blood to clot, called platelets, are another type of blood cell that can be affected during cancer treatment, especially chemo. If platelets are low, a child might develop thrombocytopenia, a condition that leads to bleeding. This may cause a child to have small red spots on the skin, bloody or black bowel movements or vomit, or bleeding from the nose, gums, or line site (the area where fluids and medicines are routinely given to kids with cancer).

Those with a low platelet count also have to take it easy to reduce the risk of bleeding. That means avoiding rough play and contact sports (like football), and brushing with a soft toothbrush and flossing very gently. In the most extreme cases of thrombocytopenia, when the platelet count gets too low, a transfusion might be necessary.

Increased Risk of Infection

In addition to RBCs and platelets, white blood cells (WBCs) also can be depleted during or after cancer treatment. WBCs called neutrophils help fight infection and having too few can put a child at increased risk of serious infection, a condition called neutropenia. A fever can be a sign of serious infection and should be brought to your doctor's attention immediately.

Kids with neutropenia need to take special precautions against germs. Like all kids, they should wash their hands before eating, after using the bathroom, and after touching animals. But they also need to avoid crowded indoor places or visiting with friends or family members who have contagious illnesses (such as a cold, the flu, or chickenpox).

People who have recently received live-virus vaccines, such as measles or oral polio, can pass these viruses to kids with low blood cell counts, so it's also important to avoid contact with them. To prevent food-borne infection, kids with neutropenia shouldn't eat raw seafood, undercooked meat, or eggs.

Infection

Since their immune systems are compromised, kids with cancer (especially those with neutropenia) are unable to fight off bacteria and other germs that enter the body. As a result, a seasonal virus or common cold can quickly turn into a life-threatening infection.

Signs of infection include fever or chills, coughing or congestion, vomiting or diarrhea, and pain (perhaps in the ears, throat, belly, or head, or pain when going to the bathroom). Or there might be redness, swelling, pain, or oozing around the line site.

If your child has any of these symptoms, especially a fever, contact your doctor right away.

Duration of Side Effects

Most side effects go away gradually once cancer treatment ends and the healthy cells have a chance to grow again. How long this process takes typically depends on a child's overall health and the types and amounts of drugs and/or radiation he or she received.

Sometimes, however, cancer treatment can cause permanent changes to a child's growing body. These long-term side effects (called late effects) can include damage to the heart, lungs, brain, nerves, kidneys, thyroid gland, or reproductive organs. Kids may experience issues such as delayed cognitive development, growth problems, and infertility. In some cases, those who've received certain types of chemotherapy are at higher risk of developing a second type of cancer later in life.

Before treatment, the doctor will talk to you about your child's risk of late effects and what precautions can be taken prior to treatment, if any. For example, some kids who undergo treatments with fertility risks can take preventive measures like egg or sperm preservation.

Outlook

Cancer treatment has come a long way — and today about 85% of children treated for cancer recover. But in the meantime, it can be difficult to help your child cope with the sometimes painful or uncomfortable side effects of treatment. Fortunately, doctors use many therapeutic measures to make treatments more tolerable for kids.

During treatment, your child is likely to feel the emotional effects of having a serious illness — and this can leave him or her feeling angry, scared, sad, or depressed. Answer questions and help explain what's going on in an age-appropriate way to help your child deal with these strong emotions and feel more prepared for the challenges ahead.

A hospital support group, child life specialist, social worker, or psychologist from the care team also might be able to offer both you and your child comfort during this difficult time.